Apparatus and methods using mechanical resonators to enhance sensitivity in lorentz force magnetometers

ABSTRACT

A Lorentz Force magnetometer based on a mechanical resonator including a resonant, vibrating electrically conducting string or insulating fiber coated with an electrically conducting material and its response to a Lorentz Force wherein the string or fiber, fixed at two ends, is tensioned over two frets (supports) separated by a distance, L, hence, becoming mechanically resonant with high Q. The frets constrain the position of the string or fiber but not the angle it makes with the fret, thus, permitting measurement of multiple vector magnetic fields. The magnetometer can be easily manufactured in arrays with the tension and, hence, resonant frequency for each magnetometer being rapidly, sequentially, and dynamically varied through the use of, e.g., piezo/MEMS elements. If the fiber is light conducting, a compact and sensitive detector using light escaping from an aperature in the conducting material coating the fiber can be implemented.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of earlier filed,copending U.S. provisional application Ser. No. 60/191,065, filed Mar.21, 2000, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to Lorentz Force magnetometers or magneticfield sensors that use mechanical resonators to enhance sensitivityincluding a simple, small, lightweight, low-cost, andlow-power-consumption sensor that utilizes a resonant string and theLorentz Force to measure multiple vector magnetic fields.

[0004] 2. Description of the Related Art

[0005] There is increasing interest in the development of miniaturemagnetometers for mapping magnetic fields found in space, industrial,environmental, and biomedical applications. The trend is constantlytoward smaller size, lower power consumption, and lower cost for similarperformance. Toward this end, recent developments have included the useof piezoresistive cantilevers (originally developed for atomic force andscanning tunneling microscopy) and μ magnetometers (based on electrontunneling effects).

[0006] The problem with the above devices is that they require, at leastin some stages of their assembly, extensive and intricate processing.Furthermore, their sensitivities, defined as the minimum detectablefield change, are generally in the range of 1 mT to 1 μT. Therefore,there remains a need for magnetometers with increased sensitivity inwhich size, power and cost are reduced.

[0007] Devices such as the pendulum clock or, more recently, quartzcrystal resonator controlled watches use mechanical resonators toenhance detection. In both examples, the accuracy is directly linked tothe quality factor or “Q” of the resonator.

[0008] A new type of mechanical resonator magnetometer based onexcitation of a resonant bar configured in a xylophone geometry withsupports at the nodes of the first transverse node is described in U.S.Pat. No. 5,959,452, issued Sep. 28, 1999, by Givens et al which isincorporated herein by reference. The xylophone magnetometer measuresthe vectorial component of the magnetic field which lies in the plane ofthe xylophone and is perpendicular to its major axis.

[0009] The response of the xylophone magnetometer was linear to a lowfrequency magnetic field over 7 decades of range and had a noise floorbelow 1 nanotesla. The high sensitivity of this sensor was based in parton the high resonant 0(≈10,000) of the xylophone resonator (0=f₀/Δfwhere f₀ is the resonance frequency and Δf is the full width at halfmaximum of the resonance response).

[0010] While the xylophone magnetometer was a significant improvement,there remains a need for a magnetometer that can measure multiple vectormagnetic fields and whose resonant frequency can be easily anddynamically varied. Ease of manufacture particularly in arrays for usein biomedical applications, particularly in catheters, is another goal.

SUMMARY OF THE INVENTION

[0011] The invention, a new type of mechanical resonator magnetometerutilizing a resonant string and based on the response to a LorentzForce, is a significant improvement over the xylophone magnetometerdisclosed in U.S. Pat. No. 5,959,452. The invention can measure multiplevector magnetic fields, can rapidly change resonant frequency and can beeasily manufactured in arrays. All of these new features allow theinvention to be useful in certain application areas including medicalapplications as discussed below.

[0012] The mechanical resonator magnetometer of the invention comprisesan electrically conducting string or an insulating fiber coated with anelectrically conducting material wherein the fiber may be lightconducting, and means for supporting the string or fiber in tension attwo locations. When a current is inserted in the string or fiber and themagnetometer placed in a magnetic field, the resulting Lorentz Forcewill cause the string or fiber to deflect along multiple axes that canbe detected. Tension of the string or fiber can be varied using, e.g.,piezo or MEMS elements. Detection of the light conducting fiberembodiment of the invention with high sensitivity and in a compactmanner may be had by forming an aperature in the electrically conductingmaterial coating the fiber and detecting the emitted light.

[0013] In the vibrating string magnetometer of the invention, theresonant structure is the string itself. Familiar devices based on theresonant string are such musical instruments as the violin and guitar.For these devices the resonant frequency is determined by the stringmass per unit length, string length and tension applied to the string.The quality factor is more difficult to determine but is broadly relatedto energy losses due to internal friction, to the string supports, andto the air.

[0014] An important issue in maintaining a high quality factor iskeeping losses low. This directly relates to the issue of detection ofthe motion of a string which carries a current in the presence of aperpendicular magnetic field. The Lorentz Force is given by F_(L)=J×Bwhere J is the vector current and B the vector magnetic field. If thedirection of the string is along the y-direction, then J=J_(y) and theinteracting fields, B_(z) and B_(x), produce forces (and motion) alongthe x and z axes respectively. By measurement of the motions, it ispossible to measure the magnetic fields B_(z) and B_(x) simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 illustrates a schematic of the mechanical structure of theresonant string magnetometer of the invention.

[0016]FIG. 2 illustrates an embodiment that uses piezo elements tochange the string tension and, hence, the resonant frequency of themagnetometer.

[0017]FIG. 3 illustrates a MEMS embodiment of the invention for changingstring tension and, hence, the resonant frequency of the magnetometer.

[0018]FIG. 4 illustrates a detection method embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The magnetometer of the invention is based on a resonantvibrating string. The string magnetometer extends the xylophonemagnetometer of U.S. Pat. No. 5,959,452 which is itself a special caseof a Lorentz Force magnetometer that uses a mechanical resonator toenhance signal-to-noise ratio (SNR).

[0020] As shown in FIG. 1, the mechanical resonator or structure onwhich the magnetometer of the invention is based is the string 10. Whenthe string with fixed ends is stretched over two frets (supports) 12,14, separated by a distance, L, the string segment between the fretsbecomes mechanically resonant with high Q. The frets constrain theposition of the string with free slope, i.e., the angle the string makeswith the fret is not constrained.

[0021] The position of the frets can be changed to adjust the tensionand, hence, the resonant frequency of the string. Tensioning can be donerapidly by piezo or microelectromechanical (MEMS) elements. An exampleis shown in FIG. 2, where piezo elements 16, 18 support frets 12, 14over which string 10 is stretched and fixed at points, A. A voltage canthen be applied to the piezo elements causing them to expand and pushupward on the frets thereby increasing the tension in the string. Analternative is to attach the string directly to the frets and to varythe distance between the frets. FIG. 3 illustrates another option whichuses a MEMS fabricated device on a silicon substrate 20 on whichdifferent length strings 22 are placed such that the resonant frequencycan be changed by selecting the appropriate string through electricalcontacts 24.

[0022] While a large number of resonances can be excited in a singlestring, the lowest resonance of a simple string is at a frequency off₀=(T/ρ)^(0.5)/2L where L is the distance between frets, ρ is the massdensity of the string and T is the tension applied to the string. In aninstrument such as a guitar or violin the device is tuned by changingthe string tension T. The ability to vary the resonant frequency of themechanical element, i.e., the string, parametrically is one of thefeatures that distinguishes the invention from the xylophonemagnetometer of U.S. Pat. No. 5,959,452. This ability has potentiallyimportant applications some of which are described below.

[0023] If the string is a conducting wire or an insulating fiber coveredwith a metal or other electrically conducting material then a sinusoidalcurrent J can be passed along the string. This current will interactwith a vectorial magnetic field perpendicular to the string axis toproduce a Lorentz Force whose direction is perpendicular to the planecontaining the string and the magnetic field. This force will causemotions of the string at the frequency of the current for the case of asteady magnetic field, i.e., f_(mf)=0 Hz, or more generally atf_(string)=f_(current)±f_(mf) where f_(string) and f_(mf) are the stringand magnetic field frequencies respectively. When f_(string)=f₀, i.e.the fundamental resonance of the string, then the response greatlyincreases and the minimum detectable magnetic field decreases.

[0024] Another feature of the invention which distinguishes it from thexylophone device is that the string can vibrate in any directionorthogonal to its axis, hence, the magnetic field can also have anyorientation in the orthogonal plane. From a vibration point of view thisis a “degenerate” case where two independent, orthogonal directions ormodes of vibration exist.

[0025] For the pure degenerate case with no preferred direction ofvibration set by the structure all vibration directions are equivalent.For example, in a guitar, one direction could be perpendicular to theface of the guitar body and the other parallel to the face of the guitarbody both with the same frequency and Q. For a real guitar thisdegeneracy is broken by the fret geometry and two related and oftenclosely spaced resonant modes are seen usually in the configurationnormal and parallel to the instrument face. In general, the directionsof motion of the string can be controlled and each mode can be excitedby a force along its direction of motion which for the magnetometer isthe Lorentz Force.

[0026] For the resonant string magnetometer of the invention, theexcitation is produced by the Lorentz Force which is controlled by amagnetic field perpendicular to the string axis and orthogonal to thedirection of motion. For specificity, imagine a right hand coordinatesystem with the string axis lying along the y-axis in space and the xand z axes lying parallel to the horizontal and vertical axes of motion,respectively. Then a magnetic field along the x-axis will excite az-axis (vertical) motion and a magnetic field along the z-axis willexcite a x-axis (horizontal) motion. As a result, a single string candetect two orthogonal components of the external magnetic fieldsimultaneously.

[0027] The string magnetometer of the invention is well suited for awide variety of low-cost, compact applications where magnetometers arerequired. In particular, the invention can be incorporated intocatheters for cardiac catheterization and other biomedical applications.Both single sensor, two magnetic field axis devices and a catheter-basedlinear, multisensor array have been developed.

[0028] The use of these catheter magnetic fields sensors in heartcatheterization for cardiac ablation will be extremely useful. Incardiac ablation, aberrant current pathways on the heart which causeextra beats and sometimes severe arrhythmia's are removed by electricaldestruction of selected regions of the heart tissue. An important aspectof this procedure is determining the precise location of the region tobe destroyed and then assessing the degree of success of thedestruction. Current practice requires a long operation whichsuccessively induces heart fibrillation followed by ablation, followedby another induction of fibrillation. If a sensor were available tomonitor the local current distributions on the heart at the positions ofthe ablating catheter, it would be possible to locate the aberrantcurrent paths directly to guide the ablation and, hence, to improve thetreatment process.

[0029] The resonant string magnetometer of the invention has therequisite capabilities. Assuming that the catheter approaches the heartwith the catheter axis perpendicular to the heart surface, the two fielddirections and the current elements which produce them lie in the planeof the heart perpendicular to the string direction. This means that thedirection and magnitude of local surface currents on the heart at theposition of the catheter can be measured. The re-entrant loops and otheranomalous current paths which produce arrhythmia's can then be detected.The sensitivity of the string magnetometer will be adequate if thedevice has a detection threshold similar to that of the xylophonemagnetometer.

[0030] In addition, another embodiment consists of a linear arrayconsisting of several magnetometers of the invention along a single axisinside a single catheter wherein the portion of the string or fiberconnecting each magnetometer is not in tension in order to minimizemechanical coupling between magnetometers. In this case, second andhigher order magnetic field gradients can be determined with significantimprovements in spatial resolution and in rejection of distant sourcesof magnetic noise such as those found in an operating room environment.

[0031] In a second biomedical application, the magnitude and directionof biological currents in the heart can be detected from outside thebody via the magnetic fields the currents produce. Such detection hasbeen demonstrated previously using SQUID (Superconducting InterferenceDetectors). However these sensors operate at liquid helium temperaturesand require complex support facilities. Arrays of string magnetometersof the invention have the potential of being able to monitor vectorcurrents on the heart at comparable resolution but at much lower cost.

[0032] A third biomedical application allows biological currents in thebrain to be detected via the magnetic fields these currents produce. Thecurrents can be those produced by the brain during normal activity or bythe evoked response to light flashes or other sensor stimulation. Thedifficulties involved are the ability to measure the very small magneticfields produced by the brain currents (significantly smaller than in thecase of the heart) and the ability to determine the position and depthof these currents including being able to identify a local current ofinterest from the general background fields generated by other braincurrents. In this respect, the high sensitivity, small size ofindividual sensors in a string/fiber based array and the ability tobring the sensor close to the skull are potential key advantages sincespatial resolution is determined by the smaller of the sensor size andthe distance between sensor and current element being measured. Hence asmall, sensitive detector able to be place close to the skull isimportant.

[0033] A second advantage of the string/fiber sensor geometry of theinvention is the ability to configure a linear array of sensors in acatheter-like geometry as discussed above. For the heart and brainapplications the catheter “chain” can be replicated into a series ofparallel chains separated by a distance, d. The result is a flexible netof sensors, for the brain a “helmet” of magnetometers which can conformclosely to the skull. Since the flexible net conforms closely to theskull and has a known spacing between elements, it may be possible touse numerical methods to “backproject” the sensor array data to improvespatial resolution of local current distributions within the brain. Theimportant factors are the dimension of an individual sensor, theintersensor spacing and the distance from sensor to current element i.e.small element size, close spacing and a close fit to the head. If thetwo-dimensional array is placed on the surface of the chest or elsewhereon the body, such a device could be used to analyze the heart and otherorgans as well. Additionally, a multilayer array or net would permitvertical gradient detection.

[0034] The string/fiber arrays, both linear and two-dimensional asdiscussed above, have even greater capability when the tension and,hence, the frequency of each magnetometer can be varied sequentially,independently and dynamically (assuming the same string/fiber throughoutthe array) using, e.g., piezo elements as discussed above. This permitsarray processing and even use of the array as a swept frequencyanalyzer. Frequency diversity will also minimize crosstalk betweenmagnetometers.

[0035] A method for detecting string motions with high sensitivity,along both y and z axes in a compact manner suitable for use as part ofan array detection embodiment of the invention will now be described.

[0036] First, as shown in FIG. 4, assume the string is a single modeoptical fiber 26 coated with a thin conducting layer such as a gold thinfilm 28. This layer serves as a conduit for the applied current andhelps confine light inserted into the fiber at its proximal end near thesource. FIG. 4 illustrates one possible embodiment which shows lightfrom a light source 30 and current from a current source 32 insertedinto the fiber, current passing on the coating on the exterior of thefiber, and the central section of the fiber supported by supports 34, 36and tensioned for use as a sensor as described above. Emitted light 38and a current return 40 are also shown.

[0037] The new detection method is to use the light passing through thefiber for detection and not to apply any exterior light for beamdeflection detection or to use piezoelectric or other detection means.The use of light detection minimizes loss of mechanical energy to thecontacts, and simplifies and minimizes the size of the detection system.There are three distinct embodiments:

[0038] An opening/aperture 42 in the conducting layer coating the fiberis created in the active sensor region. Light 44 escapes through theopening and is then detected using a detector 44 such as a positionsensitive lateral cell optical detector, a quad or bi-cell opticaldetector or a CCD array. In the last case, differences in the responseof adjacent pixels are captured to measure temporal variations in theoptical intensity gradient of light escaping the fiber. The motion ofthis intensity profile is a measure of the motion of the fiber andthereby a measure of the magnetic field. This facilitates magnetometersensor detection since all functional elements are placed in a planarsystem with no additional external source required. If magnetometerarrays are used to map the distribution of the magnetic field and thedistribution of the magnetic field gradient, such measures can be aidedby this detection approach.

[0039] A further embodiment enhances the opening/aperture in theconducting layer coating the fiber method by creating a defect in thefiber surface (via a scratch for example) or a scattering center withinthe fiber (e.g., TiO₂ particles dispersed inside the fiber) to increasethe scattered amplitude and, hence, improve detection SNR.

[0040] A final embodiment is to use two apertures/openings in orthogonalpositions on the fiber for simultaneous measurement of two orthogonalvector components of the motions or equivalently two magnetic fieldcomponents. This aids in mapping of the vector components of themagnetic field.

I claim:
 1. A magnetometer comprising: an electrically conductingstring, the string receiving a current; and means for supporting thestring in tension at two locations; the magnetometer being placed in amagnetic field to be detected, the magnetic field being perpendicular tothe direction of the current and producing a Lorentz Force perpendicularto the string, the Lorentz Force causing deflection in the string thatcan be detected.
 2. The magnetometer as recited in claim 1 , wherein theelectrically conducting string comprises an insulating fiber coated withan electrically conducting material.
 3. The magnetometer of claim 2 ,further comprising a light source, wherein the fiber is lightconducting.
 4. The magnetometer as recited in claims 1,2, or 3 furthercomprising a means for varying the tension of the string or fiber. 5.The magnetometer as recited in claim 4 , the means for varying thetension comprising piezo elements placed under the means for supporting.6. The magnetometer as recited in claim 4 , the means for varying thetension comprising a silicon substrate containing a plurality of stringsor fibers of varying lengths, the current being switchable between thestrings or fibers.
 7. A magnetometer array comprising a plurality of themagnetometers of claims 1, 2, or 3, wherein the magnetometers are joinedend to end with the portion of the string or fiber connecting twomagnetometers not in tension.
 8. The magnetometer array as recited inclaim 7 , further comprising means for varying the tension in the stringor fiber of each magnetometer in the array.
 9. The magnetometer array asrecited in claim 8 , the means for varying the tension comprising piezoelements placed under the means for supporting.
 10. The magnetometerarray as recited in claim 8 , the means for varying the tensioncomprising a silicon substrate containing a plurality of strings orfibers of varying lengths, the current being switchable between thestrings or fibers.
 11. The magnetometer of claim 3 , further comprisingmeans for detecting the motion of the fiber.
 12. The magnetometer asrecited in claim 11 , the means for detecting comprising: a firstaperature in the conducting material on the fiber; and a detector fordetecting light escaping through the aperature.
 13. The magnetometer asrecited in claim 12 , wherein the detector comprises a positionsensitive lateral cell optical detector.
 14. The magnetometer as recitedin claim 12 , wherein the detector comprises a multi-cell opticaldetector.
 15. The magnetometer as recited in claim 12 , wherein thedetector comprises a CCD detector.
 16. The magnetometer as recited inclaim 12 , further comprising a defect in the fiber surface forincreasing scattered amplitude and, hence, signal-to-noise ratio. 17.The magnetometer as recited in claim 12 , further comprising ascattering means in the center of the fiber for increasing scatteredamplitude and, hence, signal-to-noise ratio.
 18. The magnetometer asrecited in claim 12 , further comprising a second aperature in theconducting material on the fiber, the second aperature being orthongonalto the first aperature for simultaneous measurement of two orthongonalvector components of the motion of the fiber and, hence, two magneticfield components.
 19. A method for detecting a vector magnetic fieldcomprising the steps of: supporting an electrically conducting string intension at two locations; inserting a current at one end of the stringand extracting it at the other end; placing the string in a magneticfield perpendicular to the direction of the current in the string,thereby producing a Lorentz Force perpendicular to the string, theLorentz Force causing deflection in the string; and detecting thedeflection in the string.
 20. The method as recited in claim 19 ,wherein the electrically conducting string comprises an insulating fibercoated with an electrically conducting material.
 21. The method asrecited in claims 19 or 20, further comprising the step of varying thetension of the string or fiber.
 22. A method for detecting a vectormagnetic field comprising the steps of: supporting a light conductingfiber coated with an electrically conducting material in tension at twolocations; inserting a current and light at one end of the string andextracting the current and light at the other end; placing the fiber ina magnetic field perpendicular to the direction of the current in thefiber, thereby producing a Lorentz Force perpendicular to the fiber, theLorentz Force causing deflection in the fiber; and detecting thedeflection in the fiber.
 23. The method as recited in claim 22 , furthercomprising the step of varying the tension of the fiber.
 24. The methodas recited in claim 23 , further comprising the steps of: forming anaperature in the conducting material on the fiber; and detecting thelight escaping through the aperature.
 25. A magnetometer comprising: amechanical resonator other than a bar, the resonator receiving acurrent; and means for supporting the resonator; the magnetometer beingplaced in a magnetic field to be detected, the magnetic field beingperpendicular to the direction of the current and producing a LorentzForce perpendicular to the resonator, the Lorentz Force causingdeflection in the resonator that can be detected.